1. Field of the Invention
This invention relates to a fabricating process of a photomask used in an IC process applied to semiconductor wafers. More particularly, this invention relates to a method of deriving the etching correction values for the patterns of a photomask that may utilize a photomask with a specially designed pattern layout, and to a method of fabricating a photomask based on the same. This invention adds a global etching correction value in deriving etching correction values, so as to improve the accuracy of the etching correction values.
2. Description of the Related Art
As the linewidth of IC fabrication is continuously reduced, critical dimension (CD) control is more and more important to an IC process. Generally, the factors most affecting the CD of a wafer film are the lithography process and the etching process for patterning the wafer film. The CD of the patterned photoresist layer obtained with the lithography process is called an after-development critical dimension (DCD). The CD of the pattern film obtained with the etching process using the patterned photoresist layer as a mask is called an after-etching critical dimension (ECD).
When two patterns are proximal to each other, the DCD thereof is deviated due to optical proximity effect. Hence, after the layout data of the patterns of a photomask is provided, an optical proximity correction (OPC) procedure is often needed to reduce the deviation in the DCD. On the other hand, because in an etching process the local etchant consumption over the wafer depends on the local photoresist coverage ratio, deviation of ECD from DCD in a local area depends on the local pattern configuration. This is namely the micro-loading effect. Hence, in the design stage of a photomask, an etching correction is usually made according to the local pattern configurations before the OPC is made, as described in U.S. Pat. No. 6,684,382.
However, it is discovered in real practice that even an etching correction is made according to the local pattern configurations before the OPC is made, the ECD finally obtained still deviates from the predetermined critical dimension to a certain extent.